The use of hot runner in injection molding process for manufacturing plastic products is a common manufacturing process in the plastics industry. It is well-known in any prior art that in any hot runner system, it generally involves a polymeric material which is in molten form and is being injected into the hot runner's manifold system that is linked by one or more hot runner nozzles. The heated nozzles help to maintain the plastic material in molten state, and help to guide the molten melt flow into one or more of the mold cavities. The molten melt plastic is then cooled in the shape of the mold cavity, followed by part ejection when the mold is open.
In any hot runner system, the flow of hot molten melt via the runner layout into the mold cavity plays an important part in that it affects the quality of the plastic part to be molded. Valve gated mechanisms for injection molding play an integral part in controlling the flow of molten plastics to the mold cavity. Different designs and configurations are available out there for different product and process requirements. Before we describe further on the different internal valve gate designs, it is probably interesting to remember what are the causes and effects given by a basic barrel in terms of injection molding parameters. In fact, in most of the cases, the valve gate system designs are developed to solve the cosmetic problem of the molded plastic part, weld-line control of the molded part, and adjustment of the flow rate and/or injection pressure mainly on the fixed half of the mold for the molten plastic to flow into the standard mold cavity or multi-cavities.
A typical injection molding process is divided into 2 phases, i.e. the dynamic injection phase or filling-packing phase, followed by the quasi-phase, which is also termed as post-filling phase or holding phase. The more relevant importance to this discussion for the present invention would be the dynamic phase as the post-filling phase could be corrected by any system or injection molding machines currently available in the injection molding market.
For the dynamic phase operation within the barrel of the injection-molding machine, the screw-piston movement monitoring and the plastic molding cushion within the barrel of the injection-molding machine are important factors to note. The importance of monitoring the movement of the screw-piston within the barrel of a typical standard conventional injection-molding machine must not be neglected in order to ensure repeatability in the quality of the plastic part to be produced cycle after cycle. Given the fact that each polymeric materials exhibit its own pseudo or viscous-elastic polymeric behaviour, factors like the flow-rate of the molten plastic, injection speed, injection pressure etc are all important parameters that would affect the quality of the plastic part produced. Studies have shown that by controlling the flow-rate of the molten plastic for the product to be injected, it will have an effect on the anisotropy of the final plastic part.
The importance of having plastic molding cushion within the barrel is important in any injection molding process. The purpose of a cushion is to transmit plastic pressure through the sprue, runners and gates as well as the cavity for packing a plastic part. Thus, the cushion does affect the pressure drop within a standard conventional injection-molding machine. For any conventional standard injection machine, usually there is only one (1) plastic cushion. Hence for any sequential, stack or multi-layer cavity mold with more than two (2) parting layers and running at least two (2) non-identical and complex plastic parts, the volume of material filling the first (1st) and second (2nd) parting layer would not be the same. The cushion can control the backflow of the molten plastic melt during the injection phase, hence improving the quality of the moulded part. In the current market, most of the valve gate designs do not take care of the backflow of molten plastic melt on floating half of the mold since the volume of material filling the first (1st) and second (2nd) parting layer is not the same. Hence, the design in the present invention is meant to solve the backflow issues associated with the floating half of the mold.
As described briefly in the previous paragraphs, parameters like flow-rate and pressure are very important in any injection molding process. Thus, the influence of the dynamic phase of injection on the final characteristics of the plastic part must not be neglected.
Using the well-known Bernoulli's Principle for any incompressible fluid (i.e. liquid molten plastics), the flow-rate and pressure are directly linked to each other. With an increase in the flow rate of the fluid, the velocity of the fluid will increase as well and the pressure will simultaneously decrease. This phenomenon can be expressed using Bernoulli's Principle:
            p      ρ        +                  V        2            2        +    gz    =  constant
Where “p” is pressure, “μ” is fluid density (assumed constant), “V” is flow velocity, “g” is the acceleration of gravity, and “z” is the elevation of the fluid particle. The relation applies along any particular streamline of the flow. The constant described in the equation may vary across streamlines unless it can be further shown that the fluid has zero local angular velocity, which is obviously not be the case for this present invention described here.
By applying the Bernoulli's Principle, it will also be easy to understand that a conventional valve gate nozzle in both standard and two (2) layer mold with or without a variable opening and closing of the gate will be not able to independently control the flow-rate and pressure. In a conventional injection molding application, either flow-rate or pressure could be adjusted by the injection machine itself via the barrel as well as simultaneously having the valve gate nozzle adjusting either flow-rate or pressure. Such an adjustment of flow-rate and pressure is typical in any injection molding process.
For a multi-gate system using a standard conventional valve gate nozzle, it will be more difficult to have independent flow-rate and pressure adjustment at the same time. The injection barrel could effectively cover only one action with a few different gates, and each individual gate could then be re-adjusted by the respective individual valve gates, based on the Bernoulli's Principle highlighted previously concerning the relationship between flow-rate and pressure. This would mean that it would be nearly impossible to inject simultaneously two (2) good quality parts with too high a difference in terms of weight or form, for example having more than 50% difference in weight consisting of one long and thin part and another part, which is big and thick.
Similarly, if we consider the case of sequential injection molding having two (2) mold layers, with a first (1st) nozzle being able to feed the first (1st) mold layer and a second (2nd) nozzle being able to feed the second (2nd) mold layer, it is not very difficult to understand that the pressure loss will be totally different in the second (2nd) mold layer, making the injection molding process particularly difficult. Such scenario is particularly common, especially since the adjustment of the flow-rate and pressure parameters usually works well for either one (1) of the two (2) mold layers but not for the two (2) mold layers moving at the same time simultaneously or sequentially. Such a scenario could not be easily solved on any standard conventional injection-molding machine. However, there are other injection molding systems available out there to solve such specific requirements but costs play an important influencing factor in the market, and additional investments would be needed in procuring for such systems.
Various methods in the prior art out there are available to control the flow of molten plastic melt into the cavity in order to ensure good quality plastic product being produced. However it has been found that the prior art systems available out there are not able to effectively and accurately control the flow rate of the molten plastic and injection molding pressure independently in order to have more accurate and precise control of the molten plastic in the floating half of the mold on a standard conventional injection machine, especially in producing two (2) non-identical complex and intricate-shaped plastics parts at the same time. For example, most of the prior art systems which are not able to accurately control flow rate and injection pressure independently faced a strong likelihood of encountering common molding defects like weld lines, internal bubbles, flashing due to over-packing, etc. A strong weld-line can make a difference for any typical consumer plastic part under warranty. A weak weld-line could potentially be created using prior art systems with inaccurate control of the flow-rate of the molten plastic and the injection molding pressure, leading to inaccurate filling of the molten plastic, hence affecting the mechanical properties of the finished plastic part. This is especially so for multi-cavity mold where the flow of molten plastic is important. Over time, this would affect the quality of the plastic part under warranty, which would result in the quality department implementing more checks, especially if the plastic part is a basic product molded at low costs using a typical smart valve gate/pin nozzle system described in the prior art. Such additional checks will incur unnecessary additional costs and is not productive.
Therefore, the importance of balancing a hot runner system by independently adjusting the flow-rate and pressure cannot be neglected as balancing the hot runner system results in overall higher quality moulded parts moulded with uniform filling. This is especially so for multi-cavity mold running on a standard conventional injection machine which typically encounter problems in molding good quality parts with cosmetic issues related to weld-lines, internal bubbles, etc. being eliminated.
Many prior art designs and systems are being disclosed to control the flow of molten plastic into the mold cavity via the use of valve gate or valve pins.
Some representatives examples of such prior art designs for valve gates or valve pins are disclosed. Examples are U.S. Pat. Nos. 4,244,909; 5,478,230; 6,632,079; 6,884,961; 7,192,268 and 7,175,420.
Gellert's U.S. Pat. No. 4,244,909 discloses a method of transferring molten plastic melt for stack molding, consisting of a valve gate located at the stationary mold plate in alignment with another valve gate located at the moving floating half, which communicates to another back-to-back valve gate via the runner passage within the moving floating half of the mold as well. The arrangement of the back-to-back valve gates within the floating half of the stack mold with the valve gate on the fixed half of the mold are all in-line with each other in a single direction. The valve gates in U.S. Pat. No. 4,244,909 are either 100% fully open or fully close during the operation with no variable adjustment or control. Our present invention disclosed is able to independently adjust and vary the opening and closing of the valve gates, hence offering better control in molding at least two (2) different non-identical plastic parts with complex and intricate shapes and sizes.
McGrevy's U.S. Pat. No. 5,478,230 discloses a manifold system with multiple passages and its associated pistons with valve gates in a back-to-back in-line relationship, with the system designed to prevent fluid leakage when the whole manifold is heated up. The valve gates are either 100% fully open or fully close during the operation. With reference to the Bernoulli's Principle explained in the previous paragraphs, the embodiment in U.S. Pat. No. 5,478,230 will not be able to independently adjust and control the flow-rate and pressure. Moreover, the flow of the molten plastic is transferred from an inlet to a plurality of passages leading to the valve gates, rather than flowing in a uni-directional single manner towards the valve gates directly as described in the present invention. This present invention disclosed consists of valve gates arranged in a back-to-back in-line arrangement with the flow of the molten plastic melt flowing directly to the valve gates. In addition, the present invention is able to independently adjust and vary opening and closing of the valve gates, hence offering better control in molding at least two (2) different non-identical plastic parts with complex and intricate shapes and sizes.
Synventive's U.S. Pat. No. 6,632,079 disclosed a hot runner system having a hydraulic power source, a manifold for distributing material injected from said injection molding machine to a plurality of gates leading to one or more mold cavities, and a controller cum transducer (i.e. position or pressure) for individual control of material injected through the gates during injection cycle. Synventive's U.S. Pat. No. 6,632,079 is able to separately adjust and control the flow-rate and pressure, with the manifold system being included to give the user with a possibility to adjust separately the rate of melt flow to each nozzle. However, the negative aspect of the system disclosed in Synventive's U.S. Pat. No. 6,632,079 would be the complexity of the final assembly, making it difficult to be implemented on a two-layer mold. The cost of developing such a system disclosed in Synventive's U.S. Pat. No. 6,632,079 will be expensive. For example, the presence of sensors embedded inside the tool (or mold) will mean that maintenance and assembly will be difficult, hence increasing the overall maintenance and assembly costs. In addition, if the valve pin is damaged and needs to be replaced, the user would need to fully dismantle the system in order to replace the valve pin, hence increasing the downtime of the tool. Our present invention disclosed has the advantage and flexibility of being able to be implemented on the floating half of the mold for either sequential, stack or multi-layer mold with multi-cavities. In addition, the present invention disclosed eliminates the need for a controller cum transducer system since it is dynamically controlled, hence providing faster response as well as saving costs. The embodiment described in our present invention would mean that maintenance and assembly costs would be lower since the design is simpler to assemble and maintain. No sensors are required in the present invention, with the dynamic action being mechanically actuated by the motors (i.e. stepper, linear or servo) which are externally located, facilitating the user operator to easily assemble and maintain.
Okamura et al's U.S. Pat. No. 7,192,268 disclosed comprises of a manifold having a manifold channel for receiving the molten pressurised plastic melt and delivering the molten plastic melt to a nozzle channel of a hot runner system. Sensors are installed to control and adjust the amount of flow of the molten plastic melt into the mold cavity. Pressure and flow-rate are individually adjustable and easily maintained by two (2) simple multi-layer valve pins installed within the back-plate of the tool. In addition, the system described in Okamura et al's U.S. Pat. No. 7,192,268 is able to improve the quality of the production. However, the downside is that it needs a manifold system, hence making it difficult to be implemented on a two (2) layer tool. For a two (2) layer tool, one can implement another similar design in the reversed manner, but this would mean that the size of the tool would be extremely massive, hence having the need to use a bigger or extended injection-molding machine. Therefore, the system described in Okamura et al's U.S. Pat. No. 7,192,268 is suitable for single layer tool rather than two (2) parting layer tool. Our present invention disclosed has the advantage of being able to be implemented on a two-layer mold, especially on the floating half of the mold for either sequential, stack or multi-layer mold with multi-cavities without the need of a manifold. In addition, the present invention disclosed eliminates the need for a sensor system since it is dynamically controlled, hence providing faster response as well as saving costs. Similarly, our present invention has a much simpler and compact design, making it easier to assemble and maintain. Hence, assembly and maintenance costs are lower as compared to Okamura et al's U.S. Pat. No. 7,192,268.
Babin's U.S. Pat. No. 7,175,420 discloses actuated valve gates within a manifold in which the movements of the valve gates are independently actuated and can control the flow-rate and pressure for each cavity via the use of controller-sensor system. Similarly, the embodiment described in U.S. Pat. No. 7,175,420 is almost as difficult to be implemented on a two (2) parting layer mold. The embodiment described in U.S. Pat. No. 7,175,420 is for single layer tooling with two (2) valves in-line with the overall tool shown to be quite thick. Hence if the same embodiment described in U.S. Pat. No. 7,175,420 was to be applied on a two (2) parting layer tool, it would increase the overall thickness of the tooling. Moreover, the embodiment described in U.S. Pat. No. 7,175,420 proposed having an inclined valve pin situated directly inside the nozzle. This would not be appropriate for a two (2) layer mold since this would mean that the nozzle would become particularly large, hence reducing the possibility of using small pitching between the products especially in a two (2) layer mold with multi-cavities tooling. Hence the possibility of implementing the embodiment described in U.S. Pat. No. 7,175,420 on a two (2) layer mold would result in a thick tooling, making it incompatible to use with majority of standard daylight conventional injection molding machine. However, our present invention disclosed can independently adjust and control the flow-rate and pressure for the molten plastic melt with the added advantage of being able to be implemented on the floating half of the mold for either sequential, stack or multi-layer mold with multi-cavities. In addition, the present invention disclosed eliminates the need for a sensor system since it is dynamically controlled, hence providing faster response as well as saving costs. Moreover, the design disclosed in the present invention is smaller and compact as compared to Babin's U.S. Pat. No. 7,175,420, hence making it cheaper and less expensive to fabricate the mold.
Hence, to summarise, most of the systems and designs described here can easily control and vary flow-rate of the molten plastic melt into the mold cavity, be it a standard mold or sequential, stack or multi-layer mold etc. Some might even be capable of compensating for pressure drop changes. However it is not easy to balance the flow-rate and pressure drop at the same time for a particular injection mold shot, especially in molds with at least two (2) or more parting layer, e.g. in sequential mold, stack mold, multi-layer molds, etc. and for molding complex non-identical plastic parts. In addition, most of the systems and designs described in the prior art required sensors which are embedded deep inside the tool, hence making it difficult to maintain and assemble back, even affecting the valve pin replacement. Moreover most of these designs in the prior art tend to be too big when fabricated, given the limitations of the daylight in any injection molding machine, hence having an impact on the overall mold or tooling costs.